A silicon oxide film is used in various applications, as a low refractive index film. The silicon oxide film can be deposited by a vacuum evaporation method, a coating method, etc. However, in an application to e.g. glass for buildings, glass for automobiles, cathode ray tubes (CRT) or flat displays, sputtering is used in many cases, which is suitable for film deposition on a substrate having a large area.
Usually, in a case where a silicon oxide film is deposited by sputtering in an atmosphere containing oxygen by means of a Si target, it is known that the voltage changes depending upon the flow rate of oxygen to be added to the atmosphere, if the applied power is constant. FIG. 3 shows an example of the relation (the voltage change curve) between the voltage and the flow rate of oxygen added to the atmosphere in a case where a silicon oxide film is deposited by sputtering in an atmosphere containing argon and oxygen by means of a Si target. FIG. 3 is one obtained by increasing the oxygen flow rate in the atmosphere from a state where the oxygen flow rate is 0 sccm to the state where it is 80 sccm while maintaining the argon flow rate in the atmosphere to be constant at 125 sccm, and thereafter, reducing the oxygen flow rate to a state where the oxygen flow rate in the atmosphere is 0 sccm.
As shown in FIG. 3, if the oxygen flow rate is increased during the sputtering, at the initial stage, the voltage is substantially constant at a high value. However, when the oxygen flow rate reaches a certain level, the voltage decreases, and if the oxygen flow rate is further increased, the voltage becomes substantially constant at a low value. Inversely, if the oxygen flow rate is reduced, at the initial stage, the voltage is substantially constant at a low value, when the oxygen flow rate reaches a certain level, the voltage increases, and if the oxygen flow rate is further reduced, the voltage becomes substantially constant at a high value. And, in a region where the voltage changes (the transition region), the oxygen flow rate differs as between a case where the voltage is being increased and a case where the voltage is being reduced. Namely, a hysteresis takes place.
Here, in a region where the oxygen flow rate is smaller than the transition region, the deposition rate is high, but the obtainable silicon oxide film tends to be opaque, and the absorption coefficient tends to increase, whereby there will be a drawback that it is not possible to deposit a transparent film. Further, in a region where the oxygen flow rate is larger than the transition region, the obtainable silicon oxide film will be transparent, but there will be a drawback that the deposition rate is very low. On the other hand, in the transition region, there is a merit that a transparent silicon oxide film can be obtained at a high deposition rate.
However, in the transition region, due to the hysteresis, the voltage and the oxygen flow rate are not in a constant relation, whereby it is difficult to control the discharge in a stabilized condition by the voltage and the oxygen flow rate.
As a means to eliminate the influence of the hysteresis in the transition region, control by various closed loops has been proposed (for example, JP-A-5-78836, JP-A-10-8247, JP-A-11-29863).
However, in each of these cases, there is a problem that by an external influence such as abnormal discharge (arcing) or by a measurement error, the control is likely to be erroneous, and the resulting silicon oxide film is likely to be non-uniform. Especially when arcing takes place, the voltage will thereby be decreased, the deposition rate will be decreased, and further, as the oxygen flow rate will also increase, the subsequent control will be impossible. Further, in each of these cases, it is required to carry out feedback control, and the apparatus to be used tends to be accordingly expensive.
Further, WO01/27345 (hereinafter referred to as D1) discloses a method for forming a film containing SiO2 as the main component by a sputtering method in an atmosphere containing an oxidizing gas, employing a sputtering target which contains SiC and metallic Si with an atomic ratio of C to Si being from 0.5 to 0.95 and which has a density of from 2.75×103 kg/m3 to 3.1×103 kg/m3, for the purpose of obtaining a transparent silicon oxide film at a high deposition rate.
Further, JP-A-2003-13216 (hereinafter referred to as D2) discloses a method for forming a transparent thin film by a sputtering method using a reactive gas, as a method for forming a transparent thin film constantly without a hysteresis in the transition region, wherein a compound and/or a mixture containing at least two elements which differ in the transition region wherein the film forming mode changes between a metal mode and a compound mode along with the change in the concentration of the reactive gas, is used as a target.
Further, JP-A-2003-121605, JP-A-2003-121636 and JP-A-2003-121639 disclose an antireflection film, a near infrared protection film and a bandpass filter, characterized in that a low refractive index film and a high refractive index film are alternately deposited on a substrate, wherein the low refractive index film is deposited by sputtering using conductive silicon carbide as the target, and the high refractive index film is deposited by sputtering using conductive titanium oxide as the target.
However, in the method disclosed in D1, sputtering is carried out by a DC sputtering method in a region where the oxygen flow rate is larger than the transition region, whereby the deposition rate is not sufficiently high.
Further, as a result of the study on the method disclosed in D1 by the present inventors, it has been found that a hysteresis occurs in the transition region like in the case where a Si target is employed. Namely, the present inventors have found that when the method disclosed in D1 is employed, it is difficult to control the discharge by the voltage in the transition region under a stabilized condition and that it is difficult to continuously produce a uniform film in the transition region.
Further, as a result of the study on the method disclosed in D2, the present inventors have found that it is possible to deposit a transparent thin film constantly without a hysteresis in the transition region by means of DC pulses, only when the sputtering target area is small and the applied power density is small. This will be specifically described as follows.
Firstly, the present inventors have found that in a case where a sputtering target containing silicon carbide and silicon and having a ratio in number of atoms of C to Si being from 0.5 to 0.95, is employed, it is possible to form a transparent thin film constantly without a hysteresis in the transition region using DC pulses, only when the sputtering target area is small (specifically less than about 300 cm2), and it is impossible to form a film on a substrate having a large area. And, when the target area is the same, the hysteresis tends to be more likely as the power density increases (i.e. as the deposition rate increases). Accordingly, it is possible to deposit a transparent thin film constantly without a hysteresis in the transition region using DC pulses, only when the sputtering target area is small, and the applied power density is small.
In the method disclosed in D2, a sputtering target is employed which contains silicon carbide and silicon, wherein the ratio in the number of atoms of C to Si is 1. Also in this case, it is readily conceivable that a similar phenomenon will occur.
Usually, it is necessary to increase the sputtering target area, in a case where a film is deposited on a substrate having a large area or in a case where the film deposition area is increased in order to improve the productivity. D2 discloses nothing about the sputtering target area, but as mentioned above, from the study by the present inventors, it is readily conceivable that in the method disclosed in D2, a hysteresis is likely to result when the sputtering target area is large (specifically at least about 300 cm2) and when the applied power density is large. Further, it is readily conceivable that in a case where the sputtering target area is small, and the applied power density is small, the hysteresis may disappear, but there will be a problem that the deposition rate is slow, or a transparent film can not be formed.
Further, it is also conceivable that by the method disclosed in D2, the surface roughness of the obtained film tends to be large, and in a case where it is used for an optical multilayer film wherein the number of films to be deposited is large or an optical multilayer film wherein the total film thickness is thick, e.g. when it is used for a bandpass filter, the loss of directly transmitted light will be large due to formation of a haze.
Further, a conductive silicon carbide target is employed for the formation of an antireflection film, etc. as disclosed in JP-A-2003-101605, JP-a-2003-121636 and JP-A-2003-121639. However, the present inventors have found that with such a target, the deposition rate can not be made high.
Whereas, in recent years, there has been development of an optical multilayer film having various optical characteristics, such as one capable of reflecting light having a certain specific wavelength, which is obtainable by alternately depositing a silicon oxide film having a low refractive index and a transparent film such as a Nb2O5 film or a Ta2O5 film having a high refractive index. Such an optical multilayer film is obtained by depositing from a few layers to a few hundred layers of transparent films, but it can not be used if a defect such as a non-uniform portion is present even in only one layer among them. Accordingly, for the production of an optical multilayer film, it is strongly desired to develop a method which is capable of depositing a uniform film continuously and repeatedly under the same conditions and also at a high deposition rate.
Accordingly, it is an object of the present invention to provide a method for producing a silicon oxide film, whereby a transparent film having uniform optical constants such as refractive index, absorption coefficient, etc., can be deposited continuously and repeatedly under the same conditions on a substrate having a large area and at a high deposition rate, and a method for producing an optical multilayer film which has the desired performance and which can be used for various applications.